Evolution of Defects in the Bulk Structure of MoO₃ During the Catalytic Oxidation of Propene

Abstract

The evolution of characteristic defects in the bulk structure of MoO3 under propene oxidation conditions was investigated by in situ X-ray absorption spectroscopy and X-ray diffraction.Under the reaction conditions employed (273 K to 773 K, and propene to oxygen ratio from 1:1 to 1:5), orthorhombic MoO3 remains the only crystalline phase detected by XRD. The onset temperature of the reaction of propene and oxygen in the presence of MoO3 coincides with the onset of the reduction of MoO3 in He, H2 and propene (~ 620 K). At temperatures below ~720 K and independent of the atmosphere used, partial reduction of MoO3 is observed resulting in the formation of "Mo18O52" type defects in the layer structure of aMoO3. At temperatures above ~720 K and in oxygen or in an oxidizing atmosphere, the "Mo18O52" type defects are re-oxidized to MoO3. Evidently, the catalytically active molybdenum oxide phase develops under partial oxidation conditions at temperatures below 720 K and does not possess the undisturbed ideal layer structure of orthorhombic a-MoO3.The results presented clearly show the necessity and the large potential of bulk structural investigations of heterogeneous catalysts under reaction conditions. The bulk structure and particularly the type and amount of defects in the material ("real" structure) considerably affect the catalytic properties. Hence, in order to rationally design a most active heterogeneous catalyst, both structure and reactions of the surface, and structure, defects, and reactions of the bulk need to be known in detail and carefully considered.

Systematic study on the influence of the morphology of α-MoO3 in the selective oxidation of propylene

“…The samples synthesized from AHM at pH = 1 and pH = 2, which exhibited a medium catalytic activity (5-16% propylene conversion), both strongly changed their morphology and formed agglomerated plates during propylene oxidation (Figure 9d, e). This may be due to the fact that MoO3 was slightly reduced under reaction conditions forming sub-oxides [60] and -if catalytically active -also prone to structural changes. In fact, operando characterization studies on samples comparable to AHM_decomposed by…”

“…Ressler et al [60] showed that oxygen defects were formed under reaction conditions whereas α-MoO3 The particles of the sample prepared with acetic acid were thinner and shorter than the particles of MoO3•2H2O_HNO3_calc. MoO3•2H2O_HOAc_calc and AHM_pH0_24h_calc formed particles of similar size with a width of 200 nm and up to 2 µm length (see Table 2 conversion.…”

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

“…Molybdenum-based catalysts have been investigated intensively [19][20][21][22][23][24] because of their importance in the technical processes, such as propene metathesis [25,26], propene oxidation [27,28], methanol oxidation [29], ethanol oxidative photochemical reaction, and most importantly in hydrodesulfurization (HDS) processes, which represent one of the largest groups of catalytic steps in petroleum and petrochemical reactions. Recently, SBA-15-supported MoO 3 materials were used as an intermediate for the preparation of MoS 2 catalysts.…”

In situ investigations of the bulk structural evolution of vanadium-containing heteropolyoxomolybdate catalysts during thermal activation

“…containing Keggin ion and a V free Keggin ion evolves similarly during treatment in propene (Figure 8). The onset of the structural changes in the Keggin ion at ~ 573 K is again correlated to the characteristic weakening of the Mo-O bond detectable in various molybdenum oxides [29,30] which in turn coincides with the onset of catalytic activity. Both do not seem to be effected by the presence of V in the Keggin ions of H 4 [PVMo 11 O 40 ] * 13H 2 O.…”